1. Technical Field
The field of the invention relates to greyscale and color image processing and in particular to convert a high resolution greyscale image to color using a low resolution color image, for example, images acquired from satellite or aerial imaging devices with different resolutions.
2. Background Art
Any digital camera or scanner, or almost any electronic imaging device that captures image data will take in an image as either a greyscale value from 0 to 225, or a color value typically split into red, green and blue (RGB), with the intensity of each color being represented by some numeric range from 0 to 255, for example. An image with multiple spectral elements may include additional color elements, such as infrared and thermal elements and still be treated as a color image.
A greyscale image contains a range of values from 0 to 255, in the most common case. A color image or multi-spectral image contains three or more color elements represented by a range from 0 to 255. In this case, the image would actually have three bytes of information, (e.g., three 8-bit elements). One being red, one being green, and one being blue. Each pixel would have those three bytes associated with it to describe the color represented by that pixel. For example, a yellow pixel would be represented as (255, 255, 0).
Although there are many different definitions for image resolution, it is defined here as the physical pixel size used in the image, in other words, the size of the physical area a pixel covers. For example, a pixel in a 2-meter resolution image covers a 2xc3x972 square meter area, while a pixel in a 30-meter resolution image covers a 30xc3x9730 square meter area. In this case, the 2-meter resolution is called a high resolution image and the 30-meter resolution image is called a low resolution image, as compared to each other. Because of smaller pixel size, a high resolution image contains more details than a low resolution image.
According to Castleman, xe2x80x9cDigital Image Processingxe2x80x9d, 1979, pp. 119-122, incorporated herein by reference, image registration is applied to register similar images for comparison purposes. Image registration can either physically create a new image from a source image that matches the target image on a pixel by pixel basis or establish a geometric transformation for each pixel in the target image to find the location or address of the corresponding pixel from the source image.
Conversion from a color image to greyscale image is then as follows. The greyscale value is calculated directly from the three color element values: V=A*Red+B*Green+C*Blue, where Red, Green and Blue are the three color elements of a color pixel; A, B and C are coefficients that correspond to the xe2x80x9cNTSCxe2x80x9d red, green and blue CRT phosphors of 1953 and are standardized in ITU-R Recommendation BT. 601-2 (formerly CCIR Rec. 601-2), incorporated herein by reference; their values are 0.299, 0.587 and 0.114 respectively and can be adjusted according to imaging device characteristics.
Conversion from a greyscale image to color image (not pseudo color image in this case) is rather difficult without additional information. In this case, three color elements have to be calculated from a single greyscale pixel value, which is not possible mathematically.
The method of the present invention is for converting a high resolution greyscale image to color using a low resolution color or multi-spectral image of the same area.
In conjunction with present invention a low resolution color or multi-spectral image is first registered against the high resolution greyscale image. To that end, each pixel in the high resolution greyscale image has a corresponding pixel in the low resolution color image. Combining information from the registered color image and high resolution greyscale image, the method assigns color value (RGB) to each pixel in the high resolution greyscale image and converts the greyscale image to a high resolution color image.